<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title>图片缩放</title>
	</head>

	<body style="background: black;">
		<canvas id="canvas" style="display: block;margin: 0 auto;border: 1px solid #aaa;">
			你的浏览器不支持canvas。
		</canvas>
		<input type="range" id="slider" min="0.5" max="3.0" step="0.01" value="1.0" style="display: block;margin: 20px auto;width: 800px;" />
	</body>

	<script>
		var canvas = document.getElementById("canvas");
		var context = canvas.getContext("2d");

		var slider = document.getElementById("slider");

		var image = new Image(); //定义一个Image对象

		window.onload = function() {
			canvas.width = 1200;
			canvas.height = 800;

			image.src = "./images/bg.jpg"; //加载图片

			//需要等image load完再把它画到canvas中，不然的话可能有不良后果
			image.onload = function() {
				drawImageByScale(slider.value);

				slider.onmousemove = function() {
					drawImageByScale(slider.value);
				};
			};

			/**
			 * 根据一个缩放比例在画布上画一个图像
			 * @param  {float} scale 缩放比例
			 */
			function drawImageByScale(scale) {
				//根据图片的缩放比例调整图片的长宽
				var imageWidth = canvas.width * scale;
				var imageHeight = canvas.height * scale;

				//画布上的起始点：画布的大小一半减去图像大小的一半。可以为负数，表示在左上角的外面。
				var dx = canvas.width / 2 - imageWidth / 2;
				var dy = canvas.height / 2 - imageHeight / 2;

				//清空画布
				context.clearRect(0, 0, canvas.width, canvas.height);

				//把image画到canvas中，dx、dy表示起始点坐标
				context.drawImage(image, dx, dy, imageWidth, imageHeight);
			}
		};
	</script>
</html>
